Spontaneous opal glasses have been known to the glass art for many years and are produced from compositions which develop opacity as the molten batch is cooled and shaped into a glass article. The opacity is commonly the result of the growth of crystals in the glass, although other types of phase separation phenomena taking place within the glass have been observed as causing opacity therein. Where crystal growth effects the opacity, the content developed is normally quite low, generally less than about 10% by volume, such that the overall physical properties of the glass, other than optical transmission, are very slightly affected, if at all.
Opal glasses have been and are marketed commercially in the as-formed state, i.e., no glaze or other surface decorating agent being applied thereon, but consumer interest interest has been especially prevalent where opal glasses have been employed in culinary and table ware applications. The customer has required articles which combine an aesthetically pleasing appearance with long time service capability.
A highly desirable characteristic of opal glasses is a dense, uniformly milky-white appearance. A very dense opacity is sought to permit the fabrication of thin-walled and, hence, lighter weight articles, while still retaining good opacity. Such glasses, however, have frequently been subject to a lesser or greater degree to a serious problem which has been labeled "flash mark", when articles of culinary or table ware have been molded therefrom. This defect consists of an area of varying degrees of opacity near the base of such articles and constitutes a frequent source of rejection for the ware. Moreover, in view of their physical characteristics, principally coefficient of thermal expansion and low softening point, it has generally been difficult to devise suitable fired-on decorating materials.